13. [aqueous Solutions] Practice

Explanation
The polar nature of water causes water molecules to have strong forces of attraction toward each other. This means that water molecules would rather stick together than mix with vegetable oil. In other words, vegetable oil has no polar sites to which water can attach.

Explanation
Remember, in an endothermic process, energy flows into the sample; exothermic means energy exits the sample. You can always think about the opposite process -- when the sodium acetate precipitated (un-dissolved), it released heat.

Explanation
Recall that electrolytes are ions that are dissolved in solution. When ionic compounds dissolve, the ions separate from each other. This enables the solution to conduct electricity much better than fresh water.

Explanation
Recall that each of these observations (along with osmotic pressure) are known as the colligative properties of a solution. The more salt you dissolve, the stronger water's intermolecular forces become (due to attraction to the full ionic charges of salt ions), making the solution prefer the liquid phase.

Explanation
Based on the solubility curve shown, at 10 degrees, the maximum amount of sodium chloride that can be dissolved in 100 mL of water is 35 g. This means that if you were to add more than 35 g of sodium chloride to the 100 mL of water at 10 degrees, it would not dissolve and would form a precipitate.

Explanation
Remember that the line in a solubility curve represents the saturation point where no more solute will dissolve under those conditions. The area under the curve represents an unsaturated solution, while a supersaturated solution would be above the curve. Don't forget -- in order to create a supersaturated solution, a heated, saturated solution must be cooled!

Explanation
We calculate concentration in terms of molarity, or moles/Liter. To find the concentration, we need to know the number of moles of solute and the number of liters of solvent.
Starting with the solute, we know we have 20 grams of NaCl. To convert into moles, we divide by the molar mass (58.5 g/mol) to give us 0.34 moles.
We then divide by the number of liters of solvent. Since we are given mL as our value, we must slide the decimal three places to the left, giving us 0.1 L.
0.34 moles / 0.1 L = 3.4 M

Explanation
This is just like standard (i), only backwards. We know that molarity = moles / liters. To find the number of moles of solute we need to use, we plug in the values we know.
0.5 M = moles CO2 / 0.25 L
=0.125 moles CO2

We must then convert moles into grams by multiplying by the molar mass (44 g/mol), giving us 0.55 g.

Explanation
This is a dilution problem, so we must use the dilution equation:
M1 x V1 = M2 x V2
(2M) x (150 mL) = (M2) x (350 mL)
M2 = 0.86 M
The part that is most often missed is the fact that V2 is the TOTAL final volume. Since we added 200 mL to the 150 mL we already had, we have a total of 350 mL. Also note that it doesn't matter which units we use for volume as long as it's the same unit on both sides of the equation.